1. Field of the Invention
This invention generally relates to amplifying optical signals, and more particularly to a control circuit and method for regulating the gain of an optical amplifier.
2. Description of the Related Art
Rare-earth doped, optical-fiber amplifiers are used in a wide variety of communications applications to generate appropriate high-power optical signals. In the telecommunications industry, especially where multi-channel transmission systems are employed, erbium-doped fiber amplifiers (EDFAs) have proven useful because they operate at wavelengths that reduce fiber and component losses and minimize dispersion effects. EDFAs are also attractive from a telecommunications standpoint because they produce high gain with relatively little noise and demonstrate polarization independency.
The gain of an optical fiber amplifier depends on a variety of parameters including the rare-earth ion concentration, the length of the doped fiber, the radius of the fiber core, and the power of the pump laser. With respect to this last parameter, it has been shown that the simplest way to operate an amplifier is with a constant pump power. By using a constant power, the amplifier demonstrates exceptional stability characteristics because it operates in saturation (i.e., provides essentially constant power output) over a wide range of input power. Optical fiber amplifiers, however, are not without drawbacks.
The gain of the amplifier tends to vary inversely with fluctuations in input power. As a result, the mean state of inversion in the gain medium changes, which produces a different gain profile, i.e., gain tilt. Changes in channel power also adversely affect amplifier performance. Specifically, when channels are added to the system, the powers of the remaining channels drop, and when channels are switched off, the powers of the remaining channels increase. This is problematic since a channel power that changes too quickly will produce bit errors in the receiver of the channel, which degrades the performance and integrity of data transmitted in the system. In the extreme, the channel power may even fall outside the receiver dynamic range, whereupon long sequences of bit errors may be generated.
In order to overcome these drawbacks, various approaches have been developed for controlling the gain of an optical amplifier in order to produce a stable output power in each channel.
One approach involves operating the amplifier in the non-saturated or linear power regime. Under these circumstances, a higher pump power must be used in order to avoid depletion by the amplified signal. A significant amount of pump power in the gain medium is left unused, however, and the amplifying medium must therefore be made shorter, either physically or effectively through a reduction in active ion concentration. Alternatively, the pump power may be transported in a waveguide with a large cross-section. An explanation of this phenomenon is described in M. Karasek and F. W. Willems: Suppression of Dynamic Cross Saturation in Cascades of Overpumped Erbium-Doped Fiber Amplifiers, IEEE Photonics Technology Letters, Vol 10, No. 7, pp. 1036–1038, (1998).
This approach has proven to be undesirable because in order to be effective, the required pump power of a saturated amplifier is about three times greater than that required to operate the amplifier in its saturated mode. This high power requirement is unsuitable for most practical, usable systems, particularly in multi-channel telecommunications applications where more than one costly pump laser must be used to fulfill transmission requirements.
Another approach involves the transmission of an extra link control channel which adds power to the signal so that the total power remains constant. An explanation of this solution is described in Zyskind et al., Fast Link Control Protection for Surviving Channels in Multiwavelength Optical Networks, 22nd European Conference on Optical Communications ECOC'96, Oslo, Paper ThC3.6, pp 5.49–552, (1996).
This approach is undesirable because it requires the use of a high-power laser to generate the link control channel signal. More specifically, in operation, the link control channel takes over the power of nearly all channels when a majority of the channels are switched off. The non-linear interaction of the payload channels with the strong link control channel produces ghost channel signals through four-wave mixing, which interfere with the payload channels. In this way the total number of transmitted channels is limited by four-wave mixing and, in some cases, by the available laser power of the link control channel laser. Further, the total power expended constantly must be high, often higher than the total power of all the maximum number of channels that are supported over this link. This power requirement is present irrespective of the number of channels in actual use. Finally, in order to implement this approach, a very fast control circuit must be used to control the speed of the link control channel. All of these drawbacks result in increased cost and power requirements for the optical system that employs the amplifier.
Another approach involves optical self-oscillation of the amplifier, which is commonly referred to as gain-clamped operation. For an explanation of this, see Chung et al., Dynamic Performance of the All-Optical Gain-Controlled EDFA Cascade in Multi-Wavelength Add/Drop Networks, 23rd European Conference on Optical Communications ECOC 97, IEE Conference Publication No. 448, pp. 139–142, (1997).
This approach is undesirable because it requires the use of high pump power in order generate the self-oscillation. Other drawbacks include both an increase in the noise figure when the self-oscillation includes a backwards traveling wave and the need for extra optical components. Both of these increase the overall cost of the system. A further complicating drawback associated with this approach is the difficulty of choosing the desired gain since it is defined by the optical feedback.
All the above approaches are undesirable for at least the reason that they waste pump power. In the approach of the over-pumped amplifier, excessive pump power is coupled out of the transmission fiber. In the case of the link control channel, excessive pump power is used to amplify the link control signal. In the self-oscillating approach, pump power is used to build up and sustain self-oscillation.
Another common approach to the gain variations involves controlling amplifier gain through adjustments in laser pump power. FIG. 1 shows one such circuit used to implement this approach. This circuit includes an input power monitor 100 and an output power monitor 110 connected to respective ends of an optical amplifier 105. In operation, a difference circuit 115 outputs an error signal which is proportional to the input and output power of the amplifier. A regulator 120 then generates a pump control signal based on the error signal which regulates the gain of the amplifier. The regulator performs this function by amplifying the error signal as much as possible in a stable manner so that the output of the optical amplifier will be adjusted in a direction which minimizes the error signal. The amplifier may even be controlled to have infinite gain at zero frequency, which reduces the error signal to zero in steady state.
The approach taken in FIG. 1 has revealed drawbacks related to sag or bump in amplifier gain, as shown in FIG. 2. More specifically, in conventional circuits such as shown in FIG. 1, the speed of the optical amplifier limits the speed of the regulator. For stability, the regulator must be slower than the amplifier at all operating conditions, including at very low input powers where it is especially slow. The slow response time of this loop causes the gain of the amplifier to sag, at 28, especially after a sudden input power increase. Also, a bump in amplifier gain, at 29, is generated after a sudden input power drop occurs, as shown in FIG. 2. These sag and bump effects translate into compromised bit-error performance of the amplifier at the receiver.
In view of the foregoing considerations, it is apparent that there is a need for an improved control circuit and method for regulating the gain of an optical amplifier, and more specifically, for a control circuit that allows the amplifier to maintain a constant gain irrespective of fluctuations in input power while avoiding the drawbacks of the previous approaches discussed above.